One issue with milling at subterranean locations is to control the amount of weight on the mill. If too much weight is applied the cutting elements or the blade cutting structure can break off. This can slow the milling progress and force a trip out of the hole to redress the mill. Too little weight applied to the mill will slow the milling penetration rate. Some mills with delicate cutting structure or in certain situations where very hard material such as INCONEL is to be milled, need control on the weight applied during milling. One such mill is described in US Publication 2011/0240367.
Another issue when milling rounded shapes such as balls of isolation valve or curved flappers is the need to centralize the mill as it tries to penetrate the rounded surface. A centralizing device would be an advantage to keeping the mill on track and finishing the milling sooner.
Past designs have recognized the benefit of applying preload to such devices as threaded connections as illustrated in US Publication 2007/0176424. Other designs have employed a stack of Belleville washers to change the length of a telescoping joint associated with a downhole mud motor and its effective operation as discussed in US Publication 2007/0000696.
What is needed and provided by the present invention is a simply way to control how much force is applied to a mill when it is operating. The design is modular to allow variation in the applied force for milling. The mill will contact the material to be milled first. As weight is applied to the mill, the Bellville washer stacks will compress until the surrounding sleeve or shoe contacts the material to be milled. Once the surrounding sleeve or shoe makes contact, additional weight can be applied, but only the predetermined weight via the Bellvilles will be transferred to the mill. As the mill advances the Belleville washers maintain the predetermined force until mill movement allows them to fully relax.
Ideally enough Bellville washers in the modules should be stacks to allow milling to be completed with first milling assembly. If this is not possible, the milling assembly can be removed from the well and a shorter skirt or variation in Bellville washer stacks is installed in the next milling assembly to allow for further penetration into material to be milled. The use of the surrounding sleeve to land on the piece being cut and its placement near the cutting location allows it to act as a centralizer for the mill as the mill operates. Those skilled in the art will more readily appreciate other aspects of the invention from a review of the detailed description of the preferred embodiment and the associated drawings while recognizing that the full scope of the invention is to be found in the appended claims.